In pressure reading devices, it is desirable to disengage the pressure indicator when the apparatus inflates or deflates a tire or other pneumatic device. In effect, during inflation or deflation, the parameter being measured is the sum of the static pressure and the dynamic pressure; of course, this sum is inherently different from the static pressure which alone interests the user. Only the static pressure reflects in effect the pressure which is present in the pneumatic device.
In apparatus with electrical measurement means, interrupters or commutators are sometimes provided to disengage the indicator when the inflating or deflating element is activated. However, experience shows that it is difficult in such systems to effect automatic cut-off of the indicator.
Some systems operate to disengage not the indicator directly, but the measurement element itself, again using commutators or interrupters which are designed to be as automatic as possible. Thus, when neither the inflating or deflating elements are activated, the electrical circuit for measuring the fluid pressure will be functioning. As a result, in such systems, this circuit operates substantially continuously, except during brief inflating and deflating operations. This results in excessive electric power consumption as well as frequent battery changes. Furthermore, to cut the circuit when the inflater is not in use would necessitate the use of a separate commutator, thus tending to negate the simplicity of the system.
Some systems provide for setting the pressure gauge at surrounding atmospheric pressure when the apparatus is activated. This is easily achievable when inflating because there is a great difference in the pressure between the system being measured and the atmosphere at the end of the inflating process. However, this leads to efforts which are too great and not very practical when deflating because such pressure difference is usually not great enough.
The present invention eliminates the above-described shortcomings by the purely mechanical apparatus as described below.